The present invention relates to a communication device in a radio communication system, and a built-in antenna for a radio communication device.
The present invention relates generally to radio communication systems and, in particular, to built-in antennas which can be incorporated into portable terminals and which allow the portable terminals to communicate within different frequency bands.
The cellular telephone industry has made phenomenal strides in commercial operations in the United States, Europe and the rest of the world. Growth in major metropolitan cities has far exceeded expectations and is rapidly outstripping system capacity. If this trend continues, the effects of this industry""s growth will soon reach even the smallest markets. Innovative solutions are required to meet these increasing capacity needs as well as maintain high quality service and avoid rising prices.
Throughout the world, one important step in the advancement of radio communication systems is the change from analogue to digital transmission. Equally significant is the choice of an effective digital transmission scheme for implementing the next generation technology, e.g. time division multiple access (TDMA) as for example GSM, GPRS, D-AMPS or code division multiple access (CDMA) as for example CDMA2000, IS-95 or W-CDMA. Furthermore, it is widely believed that the next generation of Personal Communication Networks (PCNs), employing low cost, pocket-sized, cordless telephones that can be carried comfortably and used to make or receive calls and communicate with interactive data bases like the Internet in the home, office, street, car, etc., will be provided by cellular carriers using the next generation digital cellular system infrastructure as for example W-CDMA, GPRS or EDGE. To provide an acceptable level of equipment compatibility, standards have been created in various regions of the world. For example, analogue standards such as AMPS (Advanced Mobile Phone System), NMT (Nordic Mobile Telephone) and ETACS and digital standards such as D-AMPS (e.g., as specified in EIA/TIA-IS-54-B and IS-136) and GSM (Global System for Mobile Communications adopted by ETSI) have been promulgated to standardise design criteria for radio communication systems. Once created these standards tend to be reused in the same similar form, to specify additional systems. For example, in addition to the original GSM system, there also exists the DCS1800, GPRS (General Package Radio Service), EDGE (Enhanced Data rate for GSM Evolution) (specified by ETSI), PCS1900 (specified by JTC in J-STD-007), all of which are based on GSM.
The recent evolution in cellular communication services involves the adoption of additional frequency bands for use in handling mobile communication services, e.g., for Personal Communication Services (PCS). Taking the U.S. as an example, the Cellular hyperband is assigned two frequency bands (commonly referred to as the A frequency band and the B frequency band) for carrying and controlling communications in the 800 MHz region. The PCS hyperband, on the other hand, is specified in the United States to include six different frequency bands (A, B, C, D, E, F) in the 1900 MHz region. Thus, eight frequency bands are now available in any given service area of the U.S. to facilitate communication services. Certain standards have been approved for the PCS hyperband (e.g., PCS1900 (J-STD-136)), while others have been approved for the Cellular hyperband (e.g., D-AMPS (IS-136)).
Each one of the frequency bands specified for the Cellular and the PCS hyperbands is allocated a plurality of traffic channels and at least one access or control channel. The control channel is used to control or supervise the operation of the mobile station by means of information transmitted or received from the mobile stations. Such information may include incoming call signals, outgoing call signals, page signals, page response signals, location registration signals, voice channel assignments, maintenance instructions, hand-over, and cell selection or reselection instructions as a mobile station travels out of the radio coverage of one cell and into the radio coverage of another cell. The control and voice channels may operate using either analogue modulation or digital modulation.
The signals transmitted by a base station in the downlink over the traffic and control channels are received by mobile or portable terminals, each of which has at least one antenna. Historically, portable terminals have employed a number of different antennas to receive and transmit signals over the air interface. For example, monopole antennas mounted perpendicularly to a conducting surface have been found to provide good radiation characteristics, desirable drive point impedances and relatively simple construction. Monopole antennas can be created in various physical forms. For example, rod or whip antennas have frequently been used in conjunction with portable terminals. For high frequency applications where an antenna""s length is to be minimized, another choice is the helical antenna.
As described above, it is commercially desirable to offer portable terminals which are capable of operating in widely different frequency bands, e.g., bands located in 900 MHz region, 1800 MHz region, 1900 MHz region and 2100 MHz region. Accordingly, antennas which provide adequate gain and bandwidth in all above frequency bands will need to be employed in the near future.
For example, U.S. Pat. No. 4,572,595 describes a dual-band antenna having a sawtooth-shaped conductor element. The dual band antenna is tuned to two different frequency bands. The antenna design in this patent is relatively insufficient since it is so physically close to the chassis of the mobile phone.
Japanese patent No. 6-37531 discloses a helix, which contains an inner parasitic metal rod. In this patent, the antenna can be tuned to dual resonant frequencies by adjusting the position of the metal rod. Unfortunately, the bandwidth for this design is too narrow for use in cellular communications.
Dual-band, printed, monopole antennas are known in which dual resonance is achieved by the addition of a parasitic strip in close proximity to a printed monopole antenna. While such an antenna has enough bandwidth for cellular communications, it requires the addition of a parasitic strip. Moteco AB in Sweden has designed a coil matching dual-band whip antenna and coil antenna, in which dual resonance is achieved by adjusting the coil-matching component (xc2xcxcex for 900 MHz and xc2xdxcex for 1800 MHz). This antenna has relatively good bandwidth and radiation performances and a length in the order of 40 mm. A non-uniform helical dual-band antenna which is relatively small in size is disclosed in copending, commonly assigned U.S. patent application Ser. No. 08/725 507, entitled xe2x80x9cMultiple Band Non-Uniform Helical Antennasxe2x80x9d.
Presently, antennas for radio communication devices, such as mobile phones, are mounted directly on the phone chassis. However, as the size and weight of portable terminals continue to decrease, the above-described antennas become less advantageous due to their size. Moreover, as the functionality of these future compact portable terminals increases, the need arises for built-in miniature antennas, which are capable of being resonant at multiple frequency bands.
Conventional built-in antennas currently in use in mobile phones include microstrip antennas and planar inverted-F antennas. Microstrip antennas are small in size and light in weight. The planar inverted-F antenna (PIFA) has already been implemented in a mobile phone handset, as described by Q.Kassim, xe2x80x9cInverted-F Antenna for Portable Handsetsxe2x80x9d, IEE Colloquium on Microwave filters and Antenna for personal Communication systems, pp. 3/1-3/6, February 1994, London, UK. More recently, Lai et al has published a meandering inverted-F antenna (WO 96/27219). This antenna has a size, which is about 40% of that of a conventional PIFA antenna.
FIGS. 1 and 2 illustrate the conventional planar patch antenna compared to the meandering inverted-F antenna described in Lai et al. The conventional planar patch antenna of FIG. 1 has both size and length equal to, for example, a quarter wavelength of the frequency to which the antenna is made resonant. The conventional planar antenna also has a width W. The meandering inverted-F antenna, illustrated in FIG. 2, also has a length equal to a quarter wavelength of the resonant frequency and a width equal to W; however, the size of the meandering inverted-F antenna is reduced to about 40% of the size of the conventional planar patch antenna. This reduction in size is attributable to the antenna""s meandering shape.
However, as mobile phones become smaller and smaller, both conventional microstrip antennas and PIFA antennas are still too large to fit the future small phone chassis. In copending U.S. patent application Ser. No. 09/112,366, entitled xe2x80x9cMiniature Printed Spiral Antenna for Mobile Terminalsxe2x80x9d, a printed spiral built-in antenna with a matching post was proposed. The size of the antenna was reduced to 20-30% of the conventional PIFA antenna (less than {fraction (1/10)} of the wavelength) thereby making it suitable for future mobile phones.
In addition to a reduced antenna size, next generation mobile phones will require the capability to tune to many frequency bands for cellular, wireless local area networks. In copending U.S. patent application Ser. No. 09/112,152, entitled xe2x80x9cTwin Spiral Dual Band Antennaxe2x80x9d, a multiple band, built-in antenna was proposed which is suitable for future phones. The built-in antenna comprises two spiral conductor arms, which are of different lengths, and capable of being tuned to different frequency bands. In order to increase the bandwidth of the antenna, a resistor loading technique is introduced. In another copending U.S. patent application Ser. No. 09/212 259, entitled xe2x80x9cPrinted Multi Band Antennaxe2x80x9d, a built-in patch antenna is provided which includes patch elements of different sizes and capable of being tuned to different frequency bands as can be seen in FIG. 3.
A drawback with the above described antennas is that they are still too large and they have problems tuning to multiple frequency bands while simultaneously having a broad bandwidth in each of these multiple frequency bands.
The object of the present invention is to overcome this drawback.
The above object is achieved by means of a communication device in a radio communication system, and a built-in antenna as claimed in claims 1 and 22.
Thanks to the interaction between the parasitic element and the main radiator according to claims 1 and 22, the antenna gets a very broad bandwidth at the higher frequencies.
In a preferable embodiment as claimed in claim 8, the main radiator is folded into two radiating elements, wherein one of the elements is folded approximately 180 degrees in relation to the other element. Thanks to the folding of the antenna the resonance at the higher frequency bands could be decreased in the frequency spectrum.
In another preferable embodiment of the invention, the parasitic element of the antenna is arranged in the vicinity of, and in parallel with the main radiator achieving a good interaction between the parasitic element and the main radiator.
In yet another embodiment according to claim 12, the ground pin of the parasitic element is arranged in close vicinity of the feeding pin of the main radiator achieving good matching and tuning of the antenna.
The main radiator containing the two radiating elements and the parasitic element are preferably arranged on a substrate (plastic or ceramic), said substrate being mounted on a Printed Circuit Board (PCB) as is claimed in claim 17.
In another preferable embodiment of claims 21 and 45, the folded built-in PIFA is attached to the back cover of the mobile phone in order to increase the antenna bandwidth by increasing the distance between the radiator and the printed circuit board of the phone.
Other characteristics of the invention are set out in the other dependent claims.